Rheostat



April 23, 1946. R. ABRAHAMSON RHEOSTAT Filed March 1, 1944 W0nllillihllil /////Imw llllllullllullllillllilllllll.

man/WM aux KITTOANE) Patented Apr. 23,1946

UNITED STATES PATENT OFFICE rmnos'ra'r Robert Abrahamson, Baldwin, N. Y.

Application March 1, 1944, Serial No. 524,539

2 Claims.

This invention relates to rheostats. It has particular application torotary rheostats, comprising a substantiall toroidal resistor elementrelatively to which a rotary contact arm or brush is moved in rder toconnect'a variable portion of the resistor element in a circuit,

For the purpose of this description the term rheostats is meant toinclude any variable resistor regardless of its specific use andpurpose, and is understood to include, for example, voltage dividers,also called potentiometers.

' Among its principal objects, the invention provides an improvedrheostat of compact size ca pable of carrying particularly heavycurrents, and having greater number of steps than conventional rheostatsresulting in finer and more accurate adjustment.

The invention further provides numerous features of design resulting ina more dependable rheostat for heavy currents than was heretoforeavailable. Among its features are longer life, less wear, and aparticularly low amount of heat developed at the point of contactbetween the mov. able brush or arm andthe resistor element proper.

Among the principal objects of the invention is further a. novel andimproved method of making rotary rheostats resulting in cheaperproduction and'a smaller and more reliable rheostat than conventionalmethods were capable of producing.

These and further features, advantages and details of the invention willappear more fully from a consideration of the detailed description whichfollows accompanied by drawings, showing, for purely illustrativepurposes, forms of rheostats made according to the invention, andillustrating steps in the method of making them.

Although the novel features which are believed to be characteristic ofthe invention will be particularly pointed out in the claims appendedhereto, the invention itself, its objectsand advantages, and the mannerin which it may be carried out may be better understood by referring tothe following description taken in connection with the accompanyingdrawing forming a part hereof, in which-- Fig. 1 is a perspectiveillustration of a form of mandrel for winding a helical resistorelementi Fig. 2 is a perspective illustration of a toroidal core to beinserted into the helical resistor element wound on the mandrel shown inFig. 1;

Fig. 3 is a diagrammatic illustration of the manner of placing theresistor element on the core;

Fig. 4 1s a plan view 01 a rotary rheostat made according to thisinvention.

Fig. 5 is an elevational cross-section through the rheostat, shown inFig. 4, a section being taken on line 5-5 of Fig. 4;

Fig. 6 is an elevational cross-section through a base, resistor element,and'core of a rotary rheostat designed for heavy current with andproviding a great number of steps of adjustment.

Fig. 'I is a cross-section through the resistor helix as it appearsafter removal from the mandrel;

Fig. 8 is a perspective view illustrating a method of winding a resistorhelix equipped with reenforcing contact pieces;

Fig. 9 is a greatly enlarged cross-section through a reeniorcing contactpiece, a section being taken on line 99 of Fig. 8;

Fig. 10 is a perspective view of a resistor element assembled on a core,the resistor element having reenforcing contact pieces secured toalternate convolutions of the helix; and

Fig. 11 is a cross-section through a modified form of contact piece forattachment to a resistor helix.

Similar reference characters refer to similar parts throughout thedrawing and the specificatlol'i.

In the drawing accompanying and forming part of this description theinvention is explained by reference to specific structure, but it willbe understood that the details may be modified in various respectswithout departure from the broad aspects of the invention.

I As far as I am aware, it is conventional practice to make a rotaryrheostat by first preparing a toroidal core of a suitable insulatin andheat resistant material, usually a ceramic substance, on which is thenwound the resistor element proper. This is an expensive and timeconsuming procedure due to the dii'flcuity of winding a coil on a coreextending through an arc of nearly 360. Special machines, so-cailedtoroid winders .have been developed for this purpose. However thesemachines can only be operated at relatively slow speeds. Toroid windersusuall consist of a hoopshaped bobbin which is rotatable about an axisnormal to its plane and carries a supply 01 wire. The hoop-shaped bobbinis rotated about its axis and deposits the wire from the bobbin on thecore. The portion of the core being wound extends substantially throughthe axis of rotation of the bobbin, and is slowly advanced in order tosecure the proper spacing between adjacent turns wound on the core.-Each revolution of the bobbin causes one turn or. convolution of wireto be deposited on the core.

During the winding considerable stress is exerted on the core by reasonof the tension under which the wire is necessarily maintained and becausof the deformation which the wire undergoes it is bent around the core.The stress is particularly great when heavy gauge wire or ribbon ofrectangular cross-section is wound as is commonly used in rheostats forheavy loads. This stress often leads to destruction of the highlybrittle and fragile ceramic core, and limits the gauge of wire orribbon, hereinafter referred to for convenience collectively as"resistor wire," which ma be wound on a core of a predeterminedthickness.

My improved method avoids these disadvantages. According to theinvention I prewind the resistor helix on a straight mandrel on aconventional winding machine or a lathe. The winding operation may becarried out at a much higher rate than can be attained on a toroidwinder.

A mandrel ID, as shown in Fig. 1, is equipped with a clamp H for holdingone end of the resistor wire l2 to be wound thereon. The mandrel has across-section corresponding to that of the core on which the resistorelement will be mounted later. The mandrel is rotated in the directionof the arrow i3 causing the wire to be deposited thereon in the form ofa helix H,

A core l6 of appropriate cross-section is separately prepared oi ceramicor other suitable insulating and heat resistant material.

The prewound resistor helix [4 is then removed from the straight mandreli and transferred onto the core I5, as is shown in Fig. 3. The assemblymay be provided with the usual terminal clamps shown for example in theembodiment illustrated in Fig. 5, and is secured to the core in anyconvenient manner such as by applying vitreous enamel thereto and bakingit in an oven.

A rotary rheostat so assembled is shown in Figs. 4 and 5. The rheostatcomprises a core l6 having a resistor helix l4 mounted thereonas'previously described. Helix l4 and core 15 are cemented with ceramiccement or vitreous enamel on an insulating base it of ceramic orothersuitable insulating and heat resistant material. The base has a centralaperture ll through which a shaft 18 extends. A knob or hand wheel I8 issecured to one end of the shaft and the other end of the shaft hassecured thereto a bracketed hub IS. A contact arm 20 is pivotallysecured to the bracketed portion of the hub 18 at 2! and carries a brushelement 22. The brush element 22 has a neck 22' extending through acorresponding aperture in the arm 20 and rests on the face 23 of theresistor helix it under action of a spring 24. Terminal clamps 25 and 26are connected to the ends of the resistor helix l4 and a center terminal21 leads to the hub [9 of the contact arm 20 through a spring washer 28.The helix and core assembly i4 and I may be secured to the base IS inany suitable manner, such as by fusing the parts together by a ceramicflux in an oven as is indicated at 29.

A base, a resistor element, and a core for particularly heavy currentsare illustrated in Fig.

mltting a great number of turns to be wound on a relatively short lengthof mandrel. The stress exerted on the mandrel during winding isconsiderable, and would destroy any insulating core. if it wereattempted to wind this form of helix on an insulating core directly witha toroid winder. The winding on a straight mandrel, preferably made ofhigh tensile steel, however, presents no difficulties.

The advantages of the improved form of helix shown in Fig. 7 aremanifest. A heavy current can be carried by the rheostat due to thelarge cross-sectional area of the ribbon from which the helix 3! waswound. The heat dissipation of the resistor is excellent since only itsnarrow edge is secured to the core, and a large surface area is exposedto the air. The number of steps of adjustment provided by the rheostatshown in Fig. 6 is more than twice the number which can be accommodatedon a rheostat of equal size wound in conventional manner with a ribbonresting on the core with its wide edge.

The contact surface of the resistor may be increased by machining on thehelix a flat surface 33 after completion of the assembly of the base 30,helix 3 I, and core 32. The resulting reduction of the cross-sectionalarea of the resistor ribbon at the machined portion which theoreticallywould lead to a corresponding increase in the temperature of theresistor element at the weakened portion is compensated for by theextremely favorable cooling condition for these portions which areexposed to the air and farthest remote from the base 30.

A modified form of resistor helix equipped with reenforcing contactpieces is shown in Figs. 8 to 10. Previous to the winding of theresistor wire 34 into a helix, a plurality of rcenforcing contact piecesare strung on the wire. The contact pieces 36 may be tubular pieces ofrectangular cross section and are advanced intermittently as the wire iswound on the mandrel l0, so as to cause the contact pieces to form acontiguous contact surface. The contact surface may either be on theface of the finished rheostat corresponding to surface 33 in Fig. 6, oron the periphery in the event that the contact arm or brush is designedto travel around the periphery of the resistor element. After windingthe helix is removed from the mandrel.

The reenforcing contact pieces which are preferably made of copper orbronze or brass are then metallically fused to the resistance wire orribbon in any convenient manner such as by soldering or brazing, or spotwelding.

A greatly enlarged cross-section through a contact portion 35 brazed tothe resistor wire 34 at 36 is illustrated in Fig. 9. The helix. with itsattached contact pieces, is then transferred onto a toroidal core 3'!and is fused to the core at 38. The contact pieces may then be machinedat 39 to produce a flat contact surface.

The rheostat thus made is admirably suited for carrying very heavyloads. The contact heat developed at the point of contact between theresistor element and the contact arm or brush is very low because thecontact proper is formed by two elements both of which are goodconductors. For example the contact piece 35 may be made of copper,brass, or bronze, and the brush may be made of a copper graphitecomposition. Were it not for the contact pieces of a material of highelectrical conductivity, the contact heat developed at the point ofcontact would be considerably greater leading to sparking, oxidation andan early destruction of the rheostat.

Reenforcing contact pieces may be applied to a limited number ofconvolutions of the helix if a limited number of steps of adjustment isdesired. Fig. 'shows a resistor and core assembly in which reenforcingcontact pieces 35' are secured to alternate turns of the resistor helix34' mounted on a core 40. The contact pieces are machined to produce aflat contact surface 39'. It is not necessary that the contact pieces beassembled with the resistor wire during the winding of the helix on amandrel. Fig. 11 shows in cross-section a contact piece 35" adapted forassembly with a prewound helix. The contact piece has a substantiallyU-shaped cross-section.

providing a space ll fitting over the resistor wire to which it is to besecured. The space I is shaped so as to fit the shape of the wireemployed to which it is thereafter brazed.

The features and advantages of the hereinbefore described method and therheostats produced thereby are manifold. The winding of the resistorhelix on a straight mandrel permits of higher production speeds than areobtainable with a toroid winder. Heavy gauge resistor wire may be usedfor forming the helix without danger of injury to the highly fragilecore, no appreciable force or stress being exerted on the core duringthe transfer of the prewound helix onto the core.

The winding of resistor ribbon in its narrow edge produces a resistorhelix providing a far greater number of steps of adjustment, and permitsthe use of a single rheostat where heretofore two rheostats were used inparallel because of the impossibility of winding in the conventionalmanner a heavy gauge ribbon with its narrow edge on a ceramic core.

The use of contact pieces of high electric conductivity greatly reducesthe contact heat and the consequent contact oxidation commonly occurringin conventional rheostats operated under heavy loads.

Oxidation is the chief source of destruction of rheostats since theoxides are poor conductors leading to a further increase in thedevelopment of contact heat, and since the extreme hardness of theoxides causes increased friction and a rapid wear of the resistorelement and brush.

The method of prewinding of the resistor helix on a mandrel lends itselfparticularly well to a convenient manner of securing U-shaped con- Thehinged. mounting of the contact arm in a bracketed hub insures aconstant contact pressure; which is determined by the force of thespring acting between the arm and the hub. This form of mounting makesthe brush element easily replaceable. A worn brush element may bereplaced simply by lifting the contact arm off the resistor helix. Notools are necessary for this operation and the exchange does not involvetact pieces thereto after removal of the helix from the mandrel.

the danger of permanent bending or deforming the brush arm as it sofrequently occurs in rheostats of conventional construction.

Obviously the present invention is not limited to the specificembodiments herein shown and described. It may be employed with equalbenefit in the production of other forms of rheostats. Such applicationof the invention will be obvious to persons skilled in the art, and ismanifestly within the scope and spirit of this invention.

What is claimed is:

1. A rotary rheostat comprising, in combination, a base; a toroidalresistor thereon; a central shaft extending through said base; a hubmember on said shaft; 2. brush arm hingedly mounted on said hub memberabout a hinge axis spaced from and normal to the axis of said shaft; acontact element carried by said brush arm for making contact with saidresistor; and a spring acting between said hub member and said brush armfor resiliently urging said arm and contact element against saidresistor, while permitting removal of said arm from said resistor byswinging movement of said arm away from said resistor about the hingeaxis against the action of said spring, the contact pressure beingdetermined by the stiffness of said spring.

2. A rotary rheostat comprising, in combination, a base; a toroidalresistor thereon; a central shaft extending through said base; a. hubmember on said shaft; a brush arm hingedly mounted on said hub memberabout a hinge axis spaced from and normal to the axis of said shaft; acontact element removably inserted into said brush arm for makingcontact with said resistor; and a helical spring coaxially arranged withsaid hinge axis for resiliently urging said arm and contact elementagainst said resistor, while permitting separation of said am andcontact element from said resistor by swinging movement of said arm awayfrom said resistor about the hinge axis against the action of saidspring, whereby the contact element may be conveniently removed andreplaced.

ROBERT ABRAHAMSON.

